I see that their are some misunderstandings
What is ijtihad?
Ijtihad is defined as an Islamic legal term that means "independent reasoning" or "the utmost effort an individual can put forth in an activity. By using both the Qu'ran and Hadith as resources, the scholar is required to carefully rely on analogical reasoning to find a solution to a legal problem, which is considered to be a religious duty for those qualified to conduct it. Thus, a mujtahid is recognized as an Islamic scholar who is competent in interpreting sharia by ijtihad. Today, there are many different opinions surrounding the role of ijtihad in modern society.
Is ijtihad a source of religious ruling?
yes because of the well known Hadith of Mua'd ibn Jabal (May Allah be pleased with him) which you can find in Jami' at-Tirmidi:
Some men who were companions of Mu'adh narrated from Mu'adh that the
Messenger of Allah (ﷺ) sent Mu'adh to Yemen, so he (ﷺ) said:
"How will you judge?" He said: "I will judge according to what is in
Allah's Book."
He said: "If it is not in Allah's Book ?" He said: "Then with the
Sunnah of the Messenger of Allah (ﷺ)."
He said: "If it is not in the Sunnah of Messenger of Allah (ﷺ)?"
He said: "I will give in my view."
He said: "All praise is due to Allah, the One Who made the messenger
of the Messenger of Allah suitable."
And in Sunan an-Nisa'i or an-Nasa'i you may find:
It was narrated from Shuraih that: He wrote to 'Umar, to ask him (a
question), and 'Umar wrote back to him telling him:
"Judge according
to what is in the Book of Allah. If it is not (mentioned) in the Book
of Allah, then (judge) according to the Sunnah of the Messenger of
Allah [SAW]. If it is not (mentioned) in the Book of Allah or the
Sunnah of the Messenger of Allah [SAW], then pass judgment according
to the way the righteous passed judgment. If it is not (mentioned) in
the Book of Allah, or the Sunnah of the Messenger of Allah [SAW], and
the righteous did not pass judgment concerning it, then if you wish,
go ahead (and try to work it out by yourself) or if you wish, leave
it.
And I think that leaving it is better for you. And peace be upon
you."
Now you can't add anything you want in the religion or shari'ah because this would be bid'ah to make it clear: you can't add a new kind of prayer for example saying we have Friday night prayer instead of Isha' (astaghfiru-Allah) or any kind of worship which isn't explained by Quran or Sunna. What you can add is a new Opinion a new arbitral verdict, which for example adapts a known rule to our modern time, for example: Is it allowed to read Quran on an ebookreader!
And we should follow the rules made by Mujtahid (if we accept them and know their analysis according Quran and Sunna) according to many verses in the Quran for example
And We sent not before you, [O Muhammad], except men to whom We
revealed [the message], so ask the people of the message if you do not
know.
Surat al-Anbiya' 21:7
But we have to verify -if possible- them:
O you who have believed, obey Allah and obey the Messenger and those
in authority among you. And if you disagree over anything, refer it to
Allah and the Messenger, if you should believe in Allah and the Last
Day. That is the best [way] and best in result.
Surat an-Nisa' (4:59)
This Verse show two things: we should follow what Alalh says, what our Messenger (peace be upon him) says and what our Authority says (this could be a Mujtahid, when it comes to religious matters) and the 2nd is that we have to verify if this is confirm to (our hopefully correct understanding or interpretation of the) Quran and Sunna.
A Mufti (a man who gives fatwa) is also a kind of Mujtahid but in a low level (see for example my answer here Is ijtihad open in Islam (Sunni view)?)
I hope i could help
I'm no scholar but i love to read and hope my interpretation won't miss the truth that much!
And Allah knows best
